Applied Motion BLuAC5-Q - Manual (Page 11)

11
BLuAC5 S/Q Hardware manual
920-0049 Rev. C
12/8/2014
Connecting to a host using RS-485
RS-485/422 allows you to connect more than one drive to a single host PC, PLC or other
computer. It also allows the communication cable to be long (more than 1000 feet). But the
device to which you connect must have an RS-485/422 port.
A Short tutorial on RS-485 & RS-422
There is a bit of confusion on the implementation of the these two serial interface specifica-
tions. RS-485 in the strictest definition is a “2-wire” interface that allows multi-node connec-
tions limited to “Half-duplex” serial communications. Up to 32 nodes that both transmit and
receive can be connected to the network. RS-422 also in the strictest definition is a “4-wire”
point to point connection that allows “Full-duplex” serial communications when connected to a
single node. RS-422 has one node that is the driver or transmitter and up to 10 nodes that are
receivers. RS-422 was not designed for a true multi-node network.
2- wire interfaces require one more significant feature. A network node, master or slave, must
be able to tristate its transmitter to allow other node to use the network when required. For
high speed baud rates this must be done very quickly to avoid communications collisions
4-wire interfaces can go beyond the simple point to point and do multi-node networks if the
slave nodes are capable of tri-stating their transmitters as required in the 2-wire networks.
Some RS-485 devices are setup to do this and can be used in a 4-wire configuration
On the AMP Servo drives the RS-485 can be implemented with either “2-wire” or “4-wire”. In
both cases communications is still limited to “Half-duplex” because of the nature of the serial
communications protocols used. 4-wire implementations can sometimes be easier due to the
greater number of Host RS-232 to RS-485 adaptors that support the 4-wire interface. 2-wire
implementations may require special Host adaptors that support “Auto enable” of the adaptor
transmitter. This is because PC software typically doesn’t support tri-stating the output of the
serial adaptors.
In both 2-wire and 4-wire systems one extra wire is always required to connect the “Grounds”
of all the nodes on the network. Even though in both cases the signals are sent differentially,
a command ground connection is absolutely required.
One more consideration must not be neglected. Proper cable shielding is a must. High volt-
age, high frequency, high current signals that are present on the servo motor cables can
emit a significant amount of electrical interference. Without proper shielding these signal can
disrupt even “noise tolerant” differential line drivers.